Adipic acid is a large volume commodity chemical used for the production of polymeric compounds. Cyclohexane can be used as a precursor for the manufacture of adipic acid. Of the 688,000 tons/year adipic acid capacity in the United States in 1978, 677,000 tons/year, 98%, was based on cyclohexane feedstock, Kirk-Othmer, Ency. Chem. Tech., Vol. 1, 510, John Wiley & Sons, N.Y., 1978. Because of the large volume of adipic acid required for production of polymers, increases in adipic acid yields from precursors such as cyclohexane are of significant economic importance.
Processes for the preparation of adipic acid from cyclohexane using cobalt as a catalyst are taught by U.S. Pat. No. 4,032,569; U.S. Pat. No. 4,263,453 and Japanese Pat. No. 51075018. U.S. Pat. No. 4,032,569 to Onopchenko, et al. teaches that significantly higher conversions of cyclohexane to a product predominating in adipic acid is obtained if critical amounts of cobalt are present in the reaction zone, the temperature and pressure during the reaction are maintained within critical ranges and, if the reaction is terminated within a critical time period. Critical requirements are that at least about 25 millimols of cobalt be present per mole of cyclohexane in the process and that temperature be in the range of from about 85.degree. C. to about 105.degree. C., oxygen partial pressure at least about 150 psia for a period of about 0.5 to about 3 hours. Schultz, et al., U.S. Pat. No. 4,263,453 teaches the oxidation of cyclohexane in the presence of a cobalt catalyst wherein cobalt must be present in the form of its cobaltic ion. Schulz '453 teaches that the source of the cobaltic ion is immaterial as long as the cobaltic ions are maintained in the reaction mixture during the reaction period. In order to attain high conversion, the amount of cobalt is critical and must be in excess of about 25 millimoles of cobalt per mol of cyclohexane, preferably in the range of about 50 to 100 millimols of cobalt per mol of cyclohexane Schulz '453 further notes.
As is well-known, in the oxidation of cyclohexane to adipic acid in the presence of a cobalt salt, any cobalt salt of an organic acid can be used, see U.S. Pat. No. 3,231,608 to Kollar. However, it is also well-known that if the cobaltous salt is selected as the catalyst of choice because of availability and/or solubility requirements, the cobaltous ion must be oxidized to the cobaltic ion before the reaction of the oxidation of cyclohexane to adipic acid can proceed. A period of induction therefore occurs before the period of oxidation. The induction period can be reduced by the addition of an initiator to the reaction mixture It is therefore essential that an initiator be present in the process to reduce the total process time.
Despite the considerable work performed in this area by experimenters, reported yields of adipic acid from cyclohexane have been lessened by the co-production of byproducts such as glutaric acid and succinic acid. Additionally, process times have been long, as long as 4 hours, see U.S. Pat. No. 3,231,608.
To increase the yield of adipic acid, Schulz, U.S. Pat. No. 4,263,453, teaches the reaction must be carried out in the presence of selected amounts of added free water, generally at least about 0.5 wt% of water, relative to the weight of the monobasic acid solvent, but generally no more than about 15 wt % of water relative to the solvent is taught as suitable. Reported yields of adipic acid range from 70.7 to 80.6%. Yield of glutaric acid ranges from 9.3 to 14.7%. Yield of succinic acid ranges from 9.4 to 13.9%. The water is introduced into the process at the beginning of the process, at the beginning of the period of induction. Total process time, including induction period and reaction period, is in the range of from 3.3 hours to 6.3 hours.